A measuring device includes sensor electrodes provided along a periphery of a base substrate such that a sum A of shortest distances from the sensor electrodes to an inner peripheral surface of a focus ring becomes a constant value, the sum A satisfying ∑ i = 1 N  a C i = A ( N  : the number of the sensor electrodes, Ci: measurement values and “a”: constant). A method of obtaining the amount of deviation of the central position of the measuring device in a region surrounded by the focus ring from the center of the region, includes: calculating the measurement values Ci using the measuring device; calculating the constant “a” using the measurement values Ci; calculating distances from the sensor electrodes to the inner peripheral surface of the focus ring using the constant “a” and the measurement values Ci; and calculating the amount of deviation of the central position of the measuring device based on the calculated distances.